FERMENTATION AND RESPIRATION 189 



§3. Other Kinds of Fermentation. — ^Lactic acid fermentation (the souring 

 of milk) is caused by Bacillus lactici acidi, which has the form of small paired ^ 

 rods from i.o to 1.7 micra long and from 0.3 to 0.4 micron broad. Many other ' 

 bacteria are able to produce lactic acid fermentation; such as Bacterium lactis 

 acidi, Bacillus lactis acidi, Bacterium limbatum lactis acidi. Micrococcus lactis 

 acidi, Spharococcus lactis acidi, Streptococcus acidi lactici and Bacillus acidificans 

 longissimus. 



The process of lactic acid fermentation is represented by the following 

 equation: 



Lactose Water Lactic acid* 



C12H22O11 + H2O = 4 CaHeOs. 



Besides lactic acid, acetic acid and other volatile acids usually occur jjhe amount 

 of the latter being dependent upon both the kind of bacterium and upon the com- 

 position of the nutrient mediugij Besides lactose, other kinds of sugars, such as 

 cane sugar, fructose and maltose, can be fermented to lactic acid if the proper 

 kind of bacteria is used. This fermentation occurs when milk is simply ex- 

 posed to a temperature of from 35° to 42°C. for a short time. The process 

 stops when a certain amount of acid has accumulated, but if the acidity thus 

 produced is neutrahzed with calcium carbonate fermentation begins again. - 



Lactic acid may also be obtained if a mixture of 100 g. of sugar and 10 g. of 

 casein or old cheese, in a Uter of water saturated with calcium carbonate, is al- 

 lowed to stand in an open vessel at a temperature of from 35° to 4o°C., with 

 occasional shaking. After fermentation has ceased the liquid is evaporated and 

 calcium lactate is deposited, from which free lactic acid is obtained by decompos- 

 ing the lactate with sulphuric acid. The optically inactive variety of lactic 

 acid is obtained in this process, but in some cases the optically active isomers 

 arise. When Micrococcus acidi paralactici acts in a medium containing sugar, 

 appreciable amounts of the dextro-rotatory paralactic acid are formed; Bacillus 

 acidi levolactici forms the levo-rotatory acid. pThe different powers possessed 

 by different bacteria to form optically active isomers of lactic aqid may be used 

 in identif3dng related species of these organisms ; thus, Bacterium coli commune 

 decomposes grape sugar, giving dextro-lactic acid, but Bacillus typhi abdominalis 

 produces levo-lactic acid under the same conditions.if Lactic acid bacteria have 

 been widely applied in the industries; for example, Berlin white beer is obtained 

 by the action of these forms. 



Butyric acid fermentation is produced by the bacterium, Clostridium butyri- 

 cuMf which has recently been shown to consist of a mixture of at least three 

 different species. There are also many other bacteria that produce butyric 

 acid. /f Butyric acid fermentation occurs in the complete absence of oxygen, 

 -aadr-both hydrogen and carbon dioxide always arise as gaseous products of the 

 process. The reaction is represented by the following equation : 



Glucose H.dro- Cadjon Butyric acid 



CeHiaOe = 2 H2 + 2 CO2 + CJHsOa. 



